An adaptive optics system automatically corrects for light distortions in the medium of transmission. For example, if you look far down a road on a very hot and sunny day, you will often see what is usually called a mirage. What you are seeing is the response of the rapidly changing temperature in the air causing it to act like a thick, constantly bending lens. As another example, the twinkling of stars is due to the atmosphere surrounding the Earth. Although twinkling stars are pleasant to look at, the twinkling causes blurring on an image obtained through a telescope. An adaptive optics system measures and characterizes the phase distortion of a wavefront of light as it passes through the medium of transmission (and the optical components transmitted therealong) and corrects for such phase distortion using a deformable mirror (DM) controlled in real-time by a computer. The device that measures and characterizes the phase distortions in the wavefront of light is called a wavefront sensor.
In an adaptive optics based large-aperture space telescope 11, as illustrative in FIG. 1, light from a nominal point source above the atmosphere enters the primary mirror 13 of the telescope 11 and is focused and directed by mirrors 14A and 14B to an adaptive optics subsystem 15. The adaptive optics subsystem 15 includes a tilt mirror 17 and a deformable mirror 19 disposed between its source (the mirrors 14A and 14B) and an imaging camera 31 and capturing an image of the point source. A beam splitter 21 directs a portion of the light directed to the imaging camera by the mirrors 17, 19, to a wavefront sensor 23 that measures the phase distortion in the wavefronts of light directed thereto. A computer 25 cooperates with mirror driver 27A to control the tilt mirror 17 to stabilize the image, and cooperates with the mirror driver 27B to control the deformable mirror 19 to compensate for the phase distortions measured in the wavefront of the incident light forming the image, thereby restoring sharpness of the image lost to atmospheric turbulence. In recent years, the technology and practice of adaptive optics have become well-known in the astronomical community.
The most commonly used approach in the wavefront sensor 23 is the Shack-Hartmann method. As shown in FIG. 2, this approach is completely geometric in nature and so has no dependence on the coherence of the sensed optical beam. The incoming wavefront is broken into an array of spatial samples, called subapertures of the primary aperture, by a two dimensional array of lenslets. The subaperture sampled by each lenslet is brought to a focus at a known distance F behind each array. A two dimensional detector array (e.g., such as a CCD imaging device or CMOS imaging device) captures an image of the focal spots, and computer-based image processing routine tracks lateral position of such spots. Because the lateral position of the focal spot depends on the local tilt of the incoming wavefront, a measurement of all the subaperture spot positions provides a measure of the gradient of the incoming wavefront. A computer-based two-dimensional integration process called reconstruction can then be used to estimate the shape of the original wavefront, and from the complex conjugate thereof derive the correction signals for the deformable mirror (and the tilt mirror) that compensate for the measured phase distortions.
In the Shack-Hartmann method, measurement inaccuracies due to optical distortion or misalignment of the sensor's optics are minimized by combining the received wavefront with an internal reference laser wavefront upstream of the lenslet array and measuring subaperture tilt/tip as the difference in spot position between the two waves. Since the reference wave suffers no atmospheric distortion, any displacement of the reference wave's subaperture spot position from that of the subaperture's chief ray is attributable to sensor distortion. The differential spot position between the two waves, therefore, provides an accurate measure of the received wavefront's distortion. The Shack-Hartmann sensor is more tolerant of vibration and temperature conditions which, together with its simplicity, allows it to be used in a greater number of adaptive optic applications outside of the laboratory.
However, the Shack-Hartmann method is sensitive to a phase step across the subaperture. Such a phase step may be introduced, for example, if the subaperture bridges the gap between the two segments of a mirror. If a phase step is introduced across the subaperture, the far-field spot formed by the aperture will take on the form of an unaberrated spot combined with a fringe pattern. For any given wavelength, this fringe pattern shifts with changing phase difference, but the pattern repeats for every one wavelength change in phase difference. This is commonly referred to as a 2π ambiguity in phase difference. Importantly, this 2π ambiguity leads to measurement errors for large phase steps.
In large aperture space telescopes, course adjustment is required to correct for large phase steps that are initially present within the system. As described above, the Schack-Hartmann method cannot accurately measure such large phase steps.
In addition, because the Schack-Hartmann method cannot accurately measure large phase steps, it is difficult and expensive to design and build Shack-Hartmann wavefront sensors that can operate effectively in highly turbulent transmission mediums. Such sensors require complex and costly components that provide for high sampling frequencies to ensure that the phase step between two successive sampling periods is within the dynamic range of the instrument.
Thus, there is a great need in the art for an improved wavefront sensing mechanism that avoids the shortcomings and drawbacks of prior art Schack-Hartmann wavefront sensors.